With the proliferation of broad band networks, the trend of increasing the communication speed to 10 Gbit/s or higher has been accelerated in metropolitan optical communication networks connecting cities and relay stations. In the metropolitan optical communication networks, a fiber transmission distance of 40 to 80 km has been demanded for long distance transmission. In an optical communication system for the metropolitan optical communication network, it is an important subject to reduce the size and the power consumption of the optical transmitter/receiver module.
To reduce the size and the power consumption of the optical transmitter/receiver module, a system in which a temperature control mechanism for a light emission device is not required, i.e., an uncooled system is effective. For direct modulation systems for applying an electric signal directly to a semiconductor laser and generating a light signal, materials resistant to temperature changes have been selected, and heat dissipation of device structures has been improved. As disclosed in Non-Patent Document 1 (Optical Fiber Communication Conference 2003, PD40), for example, an uncooled high speed operation of 10 Gbit/s has been attained at an operation temperature of 100° C. or higher.
In the direct modulation system described above, however, the time fluctuation of a signal light wavelength (hereinafter referred to as chirping) is large in the high speed modulation operation at a modulation speed, for example, of 10 Gbit/s or higher. Thus, a 1300 nm band with small dispersion in optical fibers has been mainly used for the signal light wavelength band. In the signal light wavelength band of 1300 nm, however, the propagation loss in the optical fiber is large. This is not suitable for long distance transmission of 30 km or more.
Generally, to transmit a high speed light signal at a modulation rate of 10 Gbit/s or more for 40 km or more, an external modulation system in which only small chirping is generated is used. Particularly, a semiconductor electro-absorption (EA) modulation device utilizing the electro-absorption effect has excellent features with respect to reduction in size, power consumption, integration ability with a semiconductor laser, etc. Particularly, a semiconductor optical integrated device in which the EA modulation device and a distributed feedback (DFB) semiconductor laser with an excellent single-wavelength property are monolithically integrated on one semiconductor substrate (hereinafter referred to as EA/DFB laser) has been used generally as a long distance transmitting light emission device for transmission over the distance of 40 km or more. In this case, the signal light wavelength of a 1550 nm band with a small transmission loss of optical fibers has been mainly used.
In order to decrease the size and the power consumption of the optical communication module for the metropolitan optical communication network, it is desirable that the EA/DFB laser be uncooled in a similar manner to the direct modulation system. For the conventional EA/DFB laser, however, a temperature control function is required for the normal operation, and an uncooled operation is impossible. The reason will be described with the operation principle of the EA modulation device.
For the normal operation of the EA/DFB laser, it is important to appropriately set a detuning (λsignal−λEA) defined as a difference between a signal light wavelength λsignal of the laser and a gain peak wavelength λEA of the EA modulation device. In a state where a voltage is not applied to the EA modulation device, the optical absorption edge wavelength of the EA modulation device is sufficiently separated from the signal light wavelength so that an optical absorption does not occur. That is, the EA modulation device is transparent to the signal light. In this case, the signal light permeates the EA modulation device and is in an ON state as the optical output. When a voltage is applied to the EA modulation device, on the other hand, the optical absorption edge wavelength of the EA modulation device shifts toward the long wavelength side through the Franz-Keldysh effect or the quantum confine Stark effect to overlap with the signal light wavelength. In this case, the signal light is absorbed to the EA modulation device and the optical output is in an OFF state.
The light intensity ratio of the ON state and the OFF state of the signal light passing through the EA modulation device and flowing out is referred to as an extinction ratio. The larger the extinction ratio is, the more it is preferred for signal transmission without an error. A high speed light signal can be generated by modulating the voltage applied to the EA modulation device at a high speed.
In the operation principle of the EA modulation device, if the detuning is excessively small, the optical absorption edge wavelength of the EA modulation device is always overlapped with the signal light wavelength. As a result, the insertion loss increases due to the increase in the fundamental absorption, whereby a sufficient optical output cannot be obtained in the ON state of the light signal to increase the bit error rate after transmission. On the other hand, in the case where the detuning is excessively large, an application voltage increases, which is required till the optical absorption edge wavelength of the EA modulation device is overlapped with the signal light wavelength. This is not suitable for a low power consumption operation. Further, in the case where the voltage applied to the FA modulation device is excessively high, wave functions of carriers leak from the quantum well, resulting in a problem of deteriorating the extinction ratio.
The detuning in the EA/DFB laser needs to be set such that the signal light is not absorbed in the ON state in which the electric field is not applied, and extinction can be sufficiently attained within a practical range of the voltage in the OFF state in which the signal light is absorbed.
On the other hand, the light emission device is demanded to normally operate at an operation temperature between −5° C. and 85° C., the light emission device being used for an optical communication module for small, low consumption power communications for the metropolitan optical communication network. However, the detuning of the EA modulation device largely varies depending on the operation temperature. This will be described with reference to FIG. 1.
FIG. 1 shows the temperature dependence of the gain peak wavelength λEA in an extent EA modulation device with a broken line and the temperature dependence of the oscillation λsignal in DFB laser with a solid line. The temperature dependence of λEA is related to the temperature dependence of the semiconductor band gap which is about +0.65 nm/° C. On the other hand, the temperature dependence of λsignal is related to the temperature dependence of the diffraction grating, which is about +0.1 nm/° C. Thus, the temperature dependence of the change of the detuning as the difference between them is about +0.55 nm/° C., and the detuning changes by about 50 nm upon the temperature change from −5° C. to 85° C. In the conventional EA/DFB laser, the fluctuation range for the detuning in which the normal operation is possible is about ±5 nm, and temperature control is applied so as to provide a predetermined characteristic, for example, at a temperature of 25° C.
Since the detuning of the EA/DFB laser, as described above, changes as much as about 50 nm, it greatly exceeds ±5 nm, which is the fluctuation range for the detuning in which the normal operation is possible. The optical loss increases at high temperature, making it difficult for ensuring the intensity of light sufficient for long distance transmission.
For solving the problem of the change of the detuning by temperature described above and attaining the uncooled system for the ED/DFB laser, the EA modulation device is set as shown by a dotted chain λ′EA in FIG. 1 such that the detuning has an appropriate value at an estimated highest working temperature, for example, at a temperature of 85° C. At a working temperature lower than the estimated highest working temperature, it is possible to adopt a method of applying an offset bias VOH to EA modulation device in accordance with the temperature change, thereby shifting the optical absorption edge wavelength of the EA modulation device and controlling it so as to always keep an appropriate detuning even when the working temperature changes. Known examples of the uncooled EA/DFB using the method include, for example, those described in Non-Patent Document 2 (Optical Fiber Communication conference 2003, PD 42) or Non-Patent Document 3 (30th ECOC 2004, Mo 4.4.7).
In the Non-Patent Documents 2, 3, a signal light wavelength in a 1300 nm band is used. In the signal light wavelength in the 1300 nm band, the optical loss is large during transmission through an optical fiber. It is not suitable for long distance transmission for a distance of 20 km or more. For long distance transmission for a distance of 40 km or more necessary for the metropolitan optical communication network, it is desirable that the operation be possible in the 1550 nm band of the signal light wavelength with a small optical loss during fiber transmission. Known examples for an uncooled EA/DFB in the 1550 nm band suitable for a long distance communication include, for example, those described in Non-Patent Document 4 (Electronics Letters, 2003, Vol. 39, No. 259), or Non-Patent Document 5 (Optical Fiber Communication Conference 2004, ThD4).
In the Non-Patent Document 4, there is no specific description about detuning and about a semiconductor quantum well structure as an optical absorption layer. Further, the Non-Patent Document 5 discloses detuning of 55 nm at a temperature of 25° C. This is a value substantially identical with that in the existent EA/DFB laser. This value does not correspond to detuning suitable for the 1550 nm band which is preferred for the long distance communication proposed in the present invention.
[Non-Patent Document 1]: Optical Fiber Communication Conference 2003, PD40
[Non-Patent Document 2]: Optical Fiber Communication Conference 2003, PD44
[Non-Patent Document 3]: 30th ECOC 2004, Mo 4.4.7
[Non-Patent Document 4]: Electronics Letters, 2003, Vol. 39, No. 25
[Non-Patent Document 5]: Optical Fiber Communication Conference 2004, ThD4